Post deposition method for regrowth of crystalline phase change material

ABSTRACT

Techniques for forming a phase change memory cell. An example method includes forming a bottom electrode within a substrate. The method includes forming a phase change layer above the bottom electrode. The method includes forming a capping layer and an insulator layer. The method includes crystallizing the phase change material in the phase change layer so that the phase change layer is void free. The method further comprises heating the phase change material in the phase change layer from the bottom electrode and as a result the phase change layer is crystallized from the bottom to the top. In one embodiment, a rapid thermal anneal (RTA) is applied for crystallizing the phase change material.

BACKGROUND

1. Field of the Invention

The present invention relates to computer memory, and more specificallyto forming substantially void free crystalline phase change material inphase change memory cells.

2. Description of Background

There are two major groups in computer memory: non-volatile memory andvolatile memory. Constant input of energy in order to retain informationis not necessary in non-volatile memory but is required in the volatilememory. Examples of non-volatile memory devices are Read Only Memory(ROM), Flash Electrical Erasable Read Only Memory, Ferroelectric RandomAccess Memory, Magnetic Random Access Memory (MRAM), and Phase ChangeMemory (PCM); non-volatile memory devices being memory in which thestate of the memory elements can be retained for days to decades withoutpower consumption. Examples of volatile memory devices include DynamicRandom Access Memory (DRAM) and Static Random Access Memory (SRAM);where DRAM requires the memory element to be constantly refreshed whileSRAM requires a constant supply of energy to maintain the state of thememory element.

The present invention is directed to phase change memory. In phasechange memory, information is stored in materials that can bemanipulated into different phases. Each of these phases exhibitdifferent electrical properties which can be used for storinginformation. The amorphous and crystalline phases are typically twophases used for bit storage (1's and 0's) since they have detectabledifferences in electrical resistance. Specifically, the amorphous phasehas a higher resistance than the crystalline phase.

Chalcogenides are a group of materials commonly utilized as phase changematerial. This group of materials contain a chalcogen (Periodic TableGroup 16/VIA) and another element. Selenium (Se) and tellurium (Te) arethe two most common elements in the group used to produce a chalcogenidesemiconductor when creating a phase change memory cell. An example ofthis would be Ge2Sb2Te5 (GST), SbTe, and In2Se3.

Altering the phase change material's state typically requires heatingthe material to a melting point and then cooling the material to one ofthe possible states. A current passed through the phase change materialcreates ohmic heating and causes the phase change material to melt. Abalance between the ohmic heating and heat flow produces a meltingregion (also referred to herein as a “switching region”) whose crosssection minimum is defined by the diameter of the bottom electrode. Inthe latter, melting and gradually cooling down the phase change materialallows time for the phase change material to form the crystalline state,while melting and abruptly cooling the phase change material quenchesthe phase change material into the amorphous state.

A problem in phase change memory is that voids may be dispersed incrystalline phase change material. Since phase change material shrinksin the crystalline phase and expands in the amorphous phase, voids canform throughout the phase change material as the phase change materialcrystallizes during cell construction. As a result, the properties ofthe phase change material, such as resistance and uniformity, becomeerratic.

SUMMARY

One example aspect of the invention is a first method for forming amemory cell. The method includes forming a bottom electrode in asubstrate, wherein the bottom electrode is a thermal conductor. Thefirst method also includes forming a phase change material layer. Thefirst method further includes forming a thermally insulating layer abovethe phase change material. The first method also includes annealing thephase change material over the bottom electrode such that the phasechange material progressively crystallizes up the via starting from thebottom electrode.

Another example aspect of the invention is a second method for forming amemory cell. The second method includes forming a bottom electrode in asubstrate, wherein the bottom electrode is a thermal conductor. Thesecond method also includes forming a via over the bottom electrode, andfilling the via with phase change material. The second method furtherincludes forming a thermally insulating layer above the phase changematerial. The thermally insulating layer has a lower heat conductivitythan the bottom electrode. The second method also includes annealing thephase change material in the via such that the phase change materialprogressively crystallizes up the via starting from the bottomelectrode.

Another example aspect of the invention is a memory cell. The memorycell includes a substrate. The memory cell includes a bottom electrodecarried by the substrate. The bottom electrode is comprised of a thermalconductor. The memory cell includes a phase change layer including phasechange material. The memory cell also includes a switching region withinthe phase change layer which is void free. The memory cell also includesa top electrode over the phase change layer.

Yet another example aspect of the invention is an apparatus for forminga memory cell. The apparatus includes a substrate. The apparatusincludes a bottom electrode carried by the substrate. The bottomelectrode is comprised of a thermal conductor. The apparatus includes aphase change layer including phase change material. The apparatusincludes an insulator layer above the phase change layer, wherein theinsulator layer is a thermal insulator. The apparatus also includes aheater configured to temporarily melt the phase change material suchthat the phase change material crystallizes without voids after melting.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1A shows a portion of a starting wafer contemplated by anembodiment of the present invention. The staring wafer includes asubstrate and a bottom electrode.

FIG. 1B shows a phase change material layer and a thermally insulatinglayer on top of the portion of the substrate.

FIG. 1C shows a capping layer formed between the phase change materialand the thermally insulating layer.

FIG. 2 shows a via formed in the substrate and over the bottomelectrode.

FIG. 3 shows the via filled, at least partially, with phase changematerial.

FIG. 4A shows a heater for annealing the phase change material.

FIG. 4B shows a thermally insulating layer and a capping layer above thephase change material.

FIG. 5 illustrates a void formed between the phase change material andthe thermally insulating layer.

FIG. 6 shows a chemical mechanical polish performed after the phasechange material is annealed.

FIG. 7 shows a memory cell with top and bottom electrodes, and void freephase change material.

FIGS. 8-11 show various methods for forming a memory cell, ascontemplated by the present invention.

DETAILED DESCRIPTION

The present invention is described with reference to embodiments of theinvention. Throughout the description of the invention reference is madeto FIGS. 1-11.

During phase change memory cell construction, the phase change materialusually becomes crystalline since the materials are typically heatedabove 250° C. As described in detail below, an embodiment of the currentinvention is a method for re-crystallizing phase change material in amemory cell. During this crystallization, voids in the switching regionof the phase change material are removed such that the phase changematerial is substantially void free. The re-crystallization of the phasechange material may be performed with a rapid thermal anneal (RTA). Therapid thermal anneal heats the phase change material so that it meltsfor a short period of time (for example, between one to five seconds).The cooling of the phase change material from the bottom electrodecauses the voids in the phase change material to migrate away from theswitching region of the phase change layer instead of being dispersedthroughout. As a result, the voids are easily removed with a chemicalmechanical polish (CMP) or with other polishing processes known to thoseskilled in the art.

In FIG. 1A, a portion of an example starting wafer 102 is shown. Thestarting wafer 102 includes a substrate 104 and the bottom electrode106. The substrate 104 may be composed of a dielectric, such as siliconoxide, silicon nitride or a combination thereof. The bottom electrode106 includes a thermally conducting and electrically conductingmaterial. Those skilled in the art will recognize that a variety ofmaterials may be utilized for the bottom electrode such as, but notlimited to, titanium nitride (TiN) and tungsten (W).

In FIG. 1B, a phase change material layer 112 is deposited over thesubstrate and electrode, followed by a thermally insulating layer 114formed above the phase change material 112. The thermally insulatinglayer 114 has a lower heat conductivity than the bottom electrode 106.The thermally insulating layer 114 is primarily used to trap heat and tocause heating and cooling of the phase change material from the bottomup (as described below). In one embodiment, the thermally insulatinglayer 114 is comprised of 80 percent ZnS (zinc sulfide) and 20 percentSiO2 (silicon dioxide).

FIG. 1C shows an alternative intermediate step. In this particularembodiment, a capping layer 116 is formed between the phase changematerial 112 and the insulating layer 114. The capping layer 116includes material that does not chemically interact with the phasechange material 112. Additionally, the capping layer 116 may be athermally insulating material. A variety of materials may be employedfor the capping layer 116 such as, but not limited to, germanium nitride(GeNx), germanium chromium nitride (GeCrNx), or silicon nitride (SiNx).

In another alternate embodiment of the invention shown in FIG. 2, a via202 is formed in the extended dielectric 204 and over the bottomelectrode 106. The via 202 may be formed by etching through thesubstrate using a wafer mask. Those skilled in the art will appreciatethat various other techniques may be used for creating the via 202. Inone embodiment, the bottom electrode 106 forms the bottom of the via202.

In FIG. 3, the via 202 is filled, at least partially, with phase changematerial 302. As discussed earlier, the phase change material 302 isprogrammable between an amorphous phase and a crystalline phase. In oneembodiment of the invention, the phase change material 302 is depositedin the via 202 is in the amorphous phase. As the phase change material302 changes from the amorphous phase to the crystalline phase duringcell construction, it shrinks. This shrinkage can cause undesirablevoids to appear in the phase change material 302. The phase changematerial 302 may be composed of, for example, Ge2Sb2Te5 (GST), SbTe, andIn2Se3. A variety of other materials may be used to from the phasechange material 302, as known to those skilled in the art.

In FIG. 4A, a thermally insulating layer 402 is shown formed above thephase change material 302. The thermally insulating layer 402 has alower heat conductivity than the bottom electrode 106. The thermallyinsulating layer 402 is primarily used to trap heat and to cause coolingof the phase change material from the bottom up (as described below). Inone embodiment, the thermally insulating layer 402 is comprised of 80percent ZnS (zinc sulfide) and 20 percent SiO2 (silicon dioxide).

FIG. 4B shows an alternative intermediate step. In this particularembodiment, a capping layer 408 is formed between the phase changematerial 302 and the insulating layer 402. The capping layer 408includes material that does not chemically interact with the phasechange material 302. Additionally, the capping layer 408 may be athermally insulating material. A variety of materials may be employedfor the capping layer 206 such as, but not limited to, germanium nitride(GeNx), germanium chromium nitride (GeCrNx), or silicon nitride (SiNx).

Turning back to FIGS. 1C and 4A, a heater 404 is shown configured totemporarily heat the wafer 102 so that the phase change material 112 and302 is void free. The heater 404 is positioned such that heat 406generated by the heater 404 is directed at least to the bottom electrode106. Since the thermally insulating layer 114 and 402 is formed abovethe phase change material 302, heat 406 generated by the heater 404heats the phase change material and, as a result, the phase changematerial 302 melts. Because of the melt flow due to capillary forces,voids normally dispersed throughout the material congregate at the topof the phase change material 302. As discussed further below, a voidfree phase change layer is formed due to re-crystallization of at leastsome of the phase change material 302. Those skilled in the art willrecognize that a variety of processes or heaters may be employed to heatthe phase change layer from the bottom to the top.

After the wafer 102 is cooled, further fabrication steps includeremoving the capping 116, 408 and insulating layers 114, 402, limitingeach cell via Reactive Ion Etching (RIE) of the phase change materialand surrounding the phase change material with dielectric and formingthe top electrode. Such fabrication techniques are known to thoseskilled in the art. These steps discussed in more detail with referenceto FIGS. 6 and 7. It should be noted that these steps may also beperformed with other invention embodiments discussed herein.

In one embodiment of the invention, which may apply to the variousarrangements shown in the Figures, the heater 404 is a laser. The laseris configured to apply at least one laser pulse to the region above thebottom electrode 106. Those skilled in the art will recognize that thestrength and wavelength of the laser pulse will be dependent on at leastthe thickness and materials employed for the phase change material 302,the capping layer 406, and the insulator layer 402.

In a particular embodiment of the invention, which is preferably appliedto cell construction including a via, the heater 404 is configured toapply a rapid thermal anneal (RTA). In this embodiment, the heater 404is configured to heat the wafer 102 to a 400° C. to 450° C. plateau. Theheater 404 is then configured to quickly heat the wafer 102 to 700° C.to 750° C. for approximately one second. Since the temperature ofcrystallization of GST is 600° C., the initial heater temperature issufficient to melt the phase change material 302. The heater 404 is thenconfigured to cool the wafer 102 for at least four seconds. This allowsenough time for the phase change material 302 to cool into a crystallinephase.

The heating process causes the phase change material 302 to anneal inthe via 202 such that the phase change material progressivelycrystallizes up the via starting from the bottom electrode. This isbecause the bottom electrode 106 is heat conducting and therefore causesthe phase change material at the bottom of the via 202 to cool fasterthan the phase change material at the top of the via 202. Conversely,the thermally insulating layer 402 causes the phase change material atthe top of the via 202 to cool slower than the phase change material atthe bottom of the via.

Since the phase change material 302 crystallizes from the bottom of thevia 202 to the top of the via 202, voids formed in the phase changematerial 302 during the initial crystallization are propagated to thetop of via 202. This is shown in FIG. 5, where a void 502 is formedbetween the phase change material 302 and the thermally insulating layer402.

At FIG. 6, a chemical mechanical polish (CMP) is performed after thephase change material 302 is annealed. The CMP removes the thermallyinsulating layer, capping layer (if used), and any voids present at thetop of the phase change material. This desirably leaves the phase changelayer void free when the phase change material is in a crystallinephase. In one embodiment of the invention, a polisher 602 is used topolish the voids from the top of the phase change material 302.

At FIG. 7, a top electrode 702 is shown formed over the phase changematerial 302 after CMP is performed. The top electrode 702 may becomposed of, for example, titanium nitride (TiN) or tungsten (W). Thus,the memory cell formed includes phase change material 302 that is voidfree in the crystalline phase between a top electrode 702 and a bottomelectrode 106.

FIG. 8 illustrates an embodiment of a method contemplated by the presentinvention for forming a memory cell. The process flow begins at formingoperation 802. During forming operation 802, the phase change layer isformed above the bottom electrode and above the substrate. As describedabove, the bottom electrode is formed within the substrate. As statedabove, the phase change layer may be in crystalline form and a varietyof materials may be used as long as the phase change material used canbe programmed to the crystalline phase or the amorphous phase. Thoseskilled in the art will recognize that a variety of processes may beutilized to deposit the phase change material in a via above the bottomelectrode including, but not limited to, atomic layer deposition (ALD)and chemical vapor deposition (CVD). After forming operation 802 iscompleted, control passes to forming operation 804.

During forming operation 804, the capping layer is formed above thephase change layer. As stated above, the capping layer is comprised ofmaterial that does not interact with the phase change layer and itshould not be a strong thermal conductor. Again, a variety of materialsmay be utilized for the capping layer. After forming operation 804 iscompleted, control passes to forming operation 806.

During forming operation 806, the insulator layer is formed above thecapping layer. The insulator layer is comprised of a thermallyinsulating material. A variety of thermally insulating materials may beemployed for the insulator layer as known to those skilled in the art.After forming operation 806 is completed control passes to annealingoperation 808.

During annealing operation 808, the phase change layer is crystallizedsuch that voids normally dispersed throughout the phase change layercongregate outside of the switching region and the resulting phasechange inside the switching region is void free. The bottom electrodeacts as a heat sink so that the phase change material is crystallizedfrom the bottom. As a result, the voids move away from the switchingregion. In the cell construction shown in FIGS. 1A-1C, the switchingregion 810 may not encompass all the phase change material in the layer.In the cell construction methods using a via, the bottom of the via isconstituted by the bottom electrode and as a result, the switchingregion reaches the lateral boundaries of the cell. After annealingoperation 808 the method ends.

Turning to FIG. 9, an alternate embodiment of the method is shown. Inthis embodiment of the method, annealing operation 808 includes applyingoperation 902. During applying operation 902, a laser pulse is appliedabove the bottom electrode. The laser heats and melts the phase changelayer, resulting in the crystallization of the phase change material toform a void free phase change layer.

FIG. 10 illustrates another embodiment of a method for forming a memorycell contemplated by the present invention. In this embodiment, whichapplies preferably to the cell construction including a via, theannealing operation 808 includes applying operation 1002. Duringapplying operation 1002, a rapid thermal anneal is utilized tocrystallize the phase change layer. In one embodiment of the method,applying operation 1002 includes a heating operation and a coolingoperation.

During heating operation, the rapid thermal anneal heats the phasechange layer to 400-450° C. plateau. After heating operation iscompleted, control passes to heating operation. During the heatingoperation, the rapid thermal anneal quickly heats the phase change layerto 700-750° C. The phase change material in the phase change layer isheated to 700-750° C. for only approximately one second. After theheating operation is completed, control passes to a cooling operation.

During the cooling operation, the phase change layer is cooled for atleast four seconds. This allows time for the phase change material inthe phase change layer to crystallize. After the cooling operation iscompleted the rapid thermal anneal is completed.

Turning to FIG. 11, yet another embodiment of the method is illustrated.In this embodiment of the invention, after annealing operation 808 iscompleted, control passes to removing operation 1102. During removingoperation 1102, the insulator layer and the capping layer are removed.Those skilled in the art will recognize that a variety of process may beemployed to remove the capping layer and the insulator layer such as,but not limited to, reactive ion etch (RIE) or wet etches such as dilutehydrofluoric acid (DHF). In one embodiment of the invention, removingoperation 1102 includes polishing operation 1104.

During polishing operation 1104, a polish is applied to the phase changelayer to remove the voids that have congregated at the top of the phasechange layer. This leaves only the void free phase change layer abovethe bottom electrode. Those skilled in the art will recognize that avariety of polishes may be utilized for polishing operation 1104including a chemical mechanical polish. After polishing operation 1104is completed removing operation 1102 is completed and control passes toforming operation 1106.

During forming operation 1106, the top electrode is formed above thevoid free phase change layer. Those skilled in the art will recognizethat a variety of processes may be utilized for forming a top electrodeincluding a metal sputter. After forming operation 1106 is completed theprocess ends.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

Having thus described the invention of the present application in detailand by reference to embodiments thereof, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

1. A method for forming a memory cell, the method comprising: forming abottom electrode in a dielectric substrate, the bottom electrode being athermal conductor; forming a layer of phase change material at leastover the bottom electrode; forming a thermally insulating layer abovethe phase change material, wherein the thermally insulating layer has alower heat conductivity than the bottom electrode; and annealing thephase change material such that the phase change material progressivelycrystallizes starting from the bottom electrode at least within aswitching region.
 2. The method of claim 1, wherein the phase changematerial is programmable between a crystalline phase and an amorphousphase.
 3. The method of claim 1, further comprising removing thethermally insulating layer after annealing the phase change material. 4.The method of claim 1, wherein annealing the phase change materialincludes heating the phase change material so that at least some of thephase change material above the bottom electrode becomes molten.
 5. Themethod of claim 4, wherein heating the phase change material includesapplying a rapid thermal anneal (RTA) process to the substrate.
 6. Themethod of claim 5, wherein the RTA process includes: heating the phasechange material to between 700° C. and 750° C. for approximately onesecond; and cooling the phase change material for at least four seconds.7. The method of claim 4, wherein heating the phase change materialincludes applying at least one laser pulse directed above the bottomelectrode.
 8. The method of claim 1, further comprising forming acapping layer between the phase change material and the insulatinglayer, the capping layer including material that does not chemicallyinteract with the phase change material.
 9. The method of claim 8,further comprising removing the capping layer after the phase changematerial crystallizes.
 10. The method of claim 1, further comprising:forming a via over the bottom electrode, wherein the bottom electrodeforms the bottom of the via; and wherein forming the layer of phasechange material includes filling the via with the phase change material.11. The method of claim 1, further comprising removing voids at the topof the phase change material after the phase change materialcrystallizes.
 12. The method of claim 11, wherein removing the voidsincludes performing a chemical mechanical polish (CMP) on the phasechange material.
 13. The method of claim 1, wherein the thermallyinsulating layer is comprised of 80 percent ZnS (zinc sulfide) and 20percent SiO2 (silicon dioxide).
 14. The method of claim 1, furthercomprising forming a top electrode above the phase change material.